feat: parse version prefix from migration filenames

Migration filenames now follow the pattern <version>_<description>.sql
(e.g. 001_initial_schema.sql). The version stored in schema_migrations
is the numeric prefix only, not the full filename stem.

Changes:
- Add ParseMigrationVersion() to extract numeric prefix from filenames
- Validate version is purely numeric, reject malformed filenames
- Rename 001.sql to 001_initial_schema.sql for clarity
- Deduplicate runMigrations/ApplyMigrations via shared applyMigrations()
- Add comprehensive unit tests for version parsing and migration apply

Closes #28

internal/database/schema/001_initial_schema.sql

@@ -0,0 +1,91 @@
+-- Migration 001: Initial schema
+-- Creates all tables for the pixa caching image proxy
+
+-- Source content blobs
+-- Files stored at: cache/src-content/<ab>/<cd>/<sha256>
+CREATE TABLE IF NOT EXISTS source_content (
+    content_hash TEXT PRIMARY KEY,
+    content_type TEXT NOT NULL,
+    size_bytes INTEGER NOT NULL,
+    fetched_at DATETIME DEFAULT CURRENT_TIMESTAMP
+);
+
+-- Source URL metadata - maps URLs to content hashes
+-- JSON stored at: cache/src-metadata/<hostname>/<path_hash>.json
+CREATE TABLE IF NOT EXISTS source_metadata (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    source_host TEXT NOT NULL,
+    source_path TEXT NOT NULL,
+    source_query TEXT NOT NULL DEFAULT '',
+    path_hash TEXT NOT NULL,
+    content_hash TEXT,
+    status_code INTEGER NOT NULL,
+    content_type TEXT,
+    response_headers TEXT,
+    fetched_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+    expires_at DATETIME,
+    etag TEXT,
+    last_modified TEXT,
+    UNIQUE(source_host, source_path, source_query),
+    FOREIGN KEY (content_hash) REFERENCES source_content(content_hash)
+);
+CREATE INDEX IF NOT EXISTS idx_source_meta_host ON source_metadata(source_host);
+CREATE INDEX IF NOT EXISTS idx_source_meta_path_hash ON source_metadata(path_hash);
+CREATE INDEX IF NOT EXISTS idx_source_meta_expires ON source_metadata(expires_at);
+CREATE INDEX IF NOT EXISTS idx_source_meta_content_hash ON source_metadata(content_hash);
+
+-- Output/transformed content blobs
+-- Files stored at: cache/dst-content/<ab>/<cd>/<sha256>
+CREATE TABLE IF NOT EXISTS output_content (
+    content_hash TEXT PRIMARY KEY,
+    content_type TEXT NOT NULL,
+    size_bytes INTEGER NOT NULL,
+    fetched_at DATETIME DEFAULT CURRENT_TIMESTAMP
+);
+
+-- Request cache - maps full request params to output content
+CREATE TABLE IF NOT EXISTS request_cache (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    cache_key TEXT NOT NULL UNIQUE,
+    source_metadata_id INTEGER NOT NULL,
+    output_hash TEXT NOT NULL,
+    width INTEGER NOT NULL,
+    height INTEGER NOT NULL,
+    format TEXT NOT NULL,
+    quality INTEGER NOT NULL DEFAULT 85,
+    fit_mode TEXT NOT NULL DEFAULT 'cover',
+    fetched_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+    access_count INTEGER NOT NULL DEFAULT 1,
+    FOREIGN KEY (source_metadata_id) REFERENCES source_metadata(id),
+    FOREIGN KEY (output_hash) REFERENCES output_content(content_hash)
+);
+CREATE INDEX IF NOT EXISTS idx_request_cache_key ON request_cache(cache_key);
+CREATE INDEX IF NOT EXISTS idx_request_cache_source ON request_cache(source_metadata_id);
+CREATE INDEX IF NOT EXISTS idx_request_cache_output ON request_cache(output_hash);
+CREATE INDEX IF NOT EXISTS idx_request_cache_fetched ON request_cache(fetched_at);
+
+-- Negative cache for failed fetches (404s, timeouts, etc.)
+CREATE TABLE IF NOT EXISTS negative_cache (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    source_host TEXT NOT NULL,
+    source_path TEXT NOT NULL,
+    source_query TEXT NOT NULL DEFAULT '',
+    status_code INTEGER NOT NULL,
+    error_message TEXT,
+    fetched_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+    expires_at DATETIME NOT NULL,
+    UNIQUE(source_host, source_path, source_query)
+);
+CREATE INDEX IF NOT EXISTS idx_negative_cache_expires ON negative_cache(expires_at);
+
+-- Cache statistics for monitoring
+CREATE TABLE IF NOT EXISTS cache_stats (
+    id INTEGER PRIMARY KEY CHECK (id = 1),
+    hit_count INTEGER NOT NULL DEFAULT 0,
+    miss_count INTEGER NOT NULL DEFAULT 0,
+    upstream_fetch_count INTEGER NOT NULL DEFAULT 0,
+    upstream_fetch_bytes INTEGER NOT NULL DEFAULT 0,
+    transform_count INTEGER NOT NULL DEFAULT 0,
+    last_updated_at DATETIME DEFAULT CURRENT_TIMESTAMP
+);
+INSERT OR IGNORE INTO cache_stats (id) VALUES (1);